This fixes the bug, it spawns evo for the correct day. For proper operation, evolution should be fixed to not spawn an extra window when it's given URLs on the command line (patch for that has already been submitted)

gnome-panel (2.14.0-0ubuntu2) dapper; urgency=low
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* debian/patches/07_clock_applet_spawn_evo_right_day.patch:
- make double clicking on a day from the panel calendar open the
evolution calendar on the same day, patch by Tollef (Ubuntu :#35167)

if you look at the 'impl_handleURI' function in 'calendar-component.c', the standard function 'time_from_isodate' is used to parse 'startdate' and 'enddate' values. the 'time_from_isodate' documentation states the string must be a Date/time value in ISO 8601 UTC format.

in other words, the date string must include the time and be in UTC. the format is also specified in the upstream of this bug: